Battery charger and charging method

ABSTRACT

A battery charger and a charging method capable of charging a battery for a short period of time while suppressing the battery temperature from rising, by incorporating in the battery charger memory a look-up table for mapping an allowable value of current with which the battery can be charged based on a battery temperature and a battery temperature rise only. The current temperature of the battery is detected (in step S 12 ) and a temperature rise is calculated from the detected temperature (in step S 14 ). An allowable current map is then retrieved from the detected temperature and the obtained temperature rise, an allowable current with which the battery can be charged while suppressing battery temperature from rising is obtained (in step S 16 ) and the battery is charged with the allowable current (in step S 20 ). Since the allowable current which the battery can be charged with, while suppressing battery temperature from rising is retrieved using the map which the allowable current is mapped, based on battery temperature and battery temperature rise, and charging current is controlled, it is possible to charge the battery for a short period of time while suppressing battery temperature from rising.

This is a continuation of application Ser. No. 09/258,140 filed Feb. 26,1999, now U.S. Pat. No. 5,075,347, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a battery charger and a charging methodfor charging a battery. The present invention relates, in particular, toa battery charger and a charging method suitable for charging a battery,such as a nickel metal hydride battery, which accumulates a large amountof heat during charging.

2. Description of the Related Art

A charging-type battery which can be repeatedly charged is recently usedas a power source for a power tool, for example. A nickel metal hydridebattery is widely used as the power tool battery of this type. To chargethe battery, a battery charger for rapidly charging the battery withhigh current is used. That is, while a battery is rapidly charged forabout 20 minutes and, a power tool can be continuously used whilereplacing a currently used battery with another one which has beencharged.

The inventor of the present invention studied ways to improve theperformance of a power tool by using, as a battery, a nickel metalhydride battery. Although the nickel metal hydride battery can providehigher capacity than a nickel-cadmium battery, a large amount of heat isgenerated during charging. If the battery temperature becomes high dueto the generated heat, the electrodes and separators of cells within thebattery deteriorate and battery life is shortened. Due to this, thenickel metal hydride battery cannot be rapidly charged with high currentunlike the nickel-cadmium battery.

Furthermore, the nickel metal hydride battery is more sensitive toovercharge than the nickel-cadmium battery and overcharging makes thebattery life shorter. For that reason, it is required to avoidovercharging the nickel metal hydride battery. As for equipment whichdoes not require battery replacement, to avoid overcharging the battery,charging current and discharging current are integrated and a battery ischarged based on the integrated value. Thus, the battery can be fullycharged. In case of charging a battery for equipment such as theabove-stated power tool which requires replacing one battery withanother, however, it has been difficult to fully charge the batterywithout overcharging it.

SUMMARY OF THE INVENTION

The invention has been made to solve the above-stated disadvantages andhas an object to provide a battery charger and a charging method capableof charging a battery for a short period of time while suppressingbattery temperature from rising.

The invention has also been made to solve the above-stated disadvantagesand has an object to provide a battery charger and a charging methodcapable of fully charging a battery without fear of an overcharge.

To attain the above object, the battery charger is technicallycharacterized by comprising:

a memory for storing a map for mapping an allowable value of current,with which a battery can be charged while suppressing batterytemperature from rising, based on a battery temperature and a batterytemperature rise;

a temperature detection device for detecting a current temperature ofthe battery;

a temperature rise output device for obtaining the temperature rise fromthe temperature detected by the temperature detection device;

an allowable current retrieval device for retrieving the map of thememory device from the temperature detected by the temperature detectiondevice and the temperature rise outputted from the temperature riseoutput device, and for obtaining the allowable value; and

a charging device for charging the battery with the allowable currentretrieved by the allowable current retrieval device.

To attain the above object, the battery charger is technicallycharacterized by comprising:

a memory for storing a map for mapping an allowable value of current,with which a battery can be charged while suppressing batterytemperature from rising, based on a battery temperature and a batterytemperature rise, the map having the allowable current value set lowwhen the battery temperature is high and set low when the temperaturerise is large;

a temperature detection device for detecting a current temperature ofthe battery;

a temperature rise output device for obtaining the temperature rise fromthe temperature detected by the temperature detection device;

an allowable current retrieval device for retrieving the map of thememory device from the temperature detected by the temperature detectiondevice and the temperature rise outputted from the temperature riseoutput device, and for obtaining the allowable value;

a charging device for charging the battery with the allowable currentretrieved by the allowable current retrieval device;

a charging completion determination device for determining that chargingis completed based on whether or not the temperature detected by thetemperature detection device and the temperature rise outputted from thetemperature rise output device belong to a region indicating a finalcharging stage (a region in which temperature rise is relatively largeand relatively low allowable current is outputted) on the map of thememory device with high frequency; and

a charging completion device for completing charging based on thedetermination of the charging completion determination device thatcharging is completed.

To attain the above object, the charging method is technicallycharacterized by comprising:

detecting current temperature of a battery;

obtaining a temperature rise from the detected temperature;

retrieving an allowable current map based on battery temperature andbattery temperature rise, from the detected temperature and the obtainedtemperature rise, and obtaining an allowable value of current, withwhich the battery can be charged while suppressing the batterytemperature from rising; and

charging the battery with the retrieved allowable current.

To attain the above object, the charging method is technicallycharacterized by comprising:

detecting a current temperature of a battery;

obtaining a temperature rise from the detected temperature;

retrieving an allowable current map based on battery temperature andbattery temperature rise, from the detected temperature and the obtainedtemperature rise, and obtaining an allowable value of current with whichthe battery can be charged while suppressing the battery temperaturefrom rising;

charging the battery with the retrieved allowable current;

determining that charging is completed based on whether the outputtedtemperature rise is relatively large and whether relatively lowallowable current is outputted from the map with a high frequency; and

completing charging based on the determination that charging iscompleted.

According to the battery charger and charging method, charging currentis controlled using a map for mapping an allowable value of current withwhich a battery can be charged while suppressing battery temperaturefrom rising based on battery temperature and battery temperature rise.That is, the map is retrieved from the battery temperature andtemperature rise. The allowable value of current, with which the batterycan be charged while suppressing the battery temperature from rising, isobtained. The battery is charged with the allowable current thusobtained. This makes it possible to charge a nickel metal hydridebattery, the temperature of which tends to rise during charging, for ashort period of time without causing deterioration due to temperaturerise. Just before charging is completed, the temperature rise of thenickel metal hydride battery is larger and the battery is charged withrelatively low current, therefore it is possible to suppress “overshoot”after the completion of charging.

According to the battery charger and charging method, charging currentis controlled using a map for mapping an allowable value of current withwhich a battery can be charged while suppressing battery temperaturefrom rising based on battery temperature and battery temperature rise.That is, the map is retrieved from the battery temperature andtemperature rise. The allowable value of current, with which the batterycan be charged while suppressing the battery temperature from rising, isobtained. Consequently, the battery is charged with the allowablecurrent thus obtained. This makes it possible to charge a nickel metalhydride battery the temperature of which tends to rise during charging,for a short period of time without causing deterioration due totemperature rise. Just before charging is completed, the temperaturerise of the nickel metal hydride battery is larger and the battery ischarged with relatively low current, therefore it is possible tosuppress “overshoot” after the completion of charging.

The completion of charging is determined, in particular, based onwhether or not a temperature rise is relatively large and whether or notrelatively low current is outputted from the map with high frequency.That is, based on whether or not temperature rise is large and whetheror not the rise is large even if charging current is decreased. Due tothis, it is possible to fully charge the battery without fear ofovercharging and without being influenced by the remaining batterycapacity, temperature and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a battery charger in an embodimentaccording to the present invention;

FIG. 2 is a perspective view of a battery pack in the embodimentaccording to the present invention;

FIG. 3 is a perspective view of a battery drill using the battery packshown in FIG. 2;

FIG. 4 is a block diagram showing a charging circuit in the batterycharger shown in FIG. 1;

FIG. 5 is an explanatory view showing the content of a map held in thecharging circuit;

FIG. 6 is a graph showing the change of charging current and that ofbattery temperature both controlled by the charging circuit; and

FIG. 7 is a flow chart showing the charging circuit process.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A battery charger and a charging method in one embodiment according tothe present invention will be described with reference to theaccompanying drawings.

The embodiment which explicitly depicts the present invention will bedescribed hereinafter with reference to the accompanying drawings.

FIG. 1 shows a battery charger 10 in one embodiment according to thepresent invention. FIG. 2 shows a battery pack 50 charged by the batterycharger. FIG. 3 shows a battery drill 70 driven by the battery pack 50.

As shown in FIG. 2, the battery pack 50, including a nickel metalhydride battery, consists of a generally cylindrical fitted section 52and a generally prismatic base 56. A key-shaped key section 54 is formedon the side surface of the fitted section 52. The first input terminalt1 connected to the cathode of the battery, the second input terminal t2connected to the anode therein and the third terminal t3 connected to atemperature sensor consisting of a thermistor are provided on the upperportions of the fitted section 52.

As shown in FIG. 1, a fitting hole 12 for fitting the fitted section 52of the battery pack 50 is formed at the battery charger 10 for chargingthe battery pack 50. A key way 14 for introducing the key section 54 ofthe fitted section 52 is formed on the sidewall of the fitting hole 12.The fitting hole 12 is integral with a housing 16 which constitutes thebattery charger 10 and is made from resin. In this embodiment, the keysection 54 is provided at the fitted section 52 of the battery pack 50and the fitted hole 12 of the battery charger 10 is provided with thekey way 14. This thereby prevents the battery pack 50 from being fittedin an incorrect manner. First to third output terminals (not shown) areprovided at the bottom of the fitting hole 12 to make contact with thefirst to third terminals t1, t2 and t3 of the battery pack 50. An LEDlamp 18 is provided on the top of the battery charger 10 to indicatethat the battery is being charged.

As shown in FIG. 3, a fitting hole 72 for fitting the fitted section 52of the battery pack 59 is provided in the battery drill 70. Whenelectric power is supplied from the first and second input terminals t1and t2 of the battery pack 50 to the battery drill 70, a motor (notshown) is driven to rotate a chuck 76. If the battery drill 70 is inuse, a plurality of batteries which have been charged within the batterypack 50 are sequentially used to allow continuous operation. To thisend, the battery charger 10 in this embodiment is designed to be capableof rapidly charging the battery pack 50 in approximately 20 minutes.

FIG. 4 shows the configuration of a control circuit within the batterycharger 10. The control circuit 30 consists of a temperature detectionsection 38 for detecting the temperature of a battery from an outputvalue of a temperature sensor (or thermistor) 56 provided at the batterypack 50 side, a memory section 39 for storing a current controlling map,a control section 34 for obtaining a temperature rise by differentiatinga temperature value outputted from the temperature detection section 38,obtaining an allowable current value with which the battery can becharged while suppressing the rise of battery temperature fromretrieving the map based on the temperature value and the temperaturerise, and outputting the allowable current value as a current commandvalue to the charging control section 34, and the charging currentcontrol section 34 for controlling a power supply circuit 32 based onthe current command value derived from the control section 36 and fromadjusting the battery charging current.

Description of the constitution of the map used for current control willcontinuously be given with reference to FIG. 5.

If the battery is charged with high current, charging time is shortenedbut temperature rises accordingly. Conversely, if the battery is chargedwith lower current, charging time is lengthened but temperature rise issmaller. The nickel metal hydride battery, in particular, possessescharacteristics that a temperature gradient (or temperature rise)greatly varies in accordance with charging current or the presentlycharged capacity. Considering the characteristics, in this embodiment,the battery is charged while changing current value to suppress thetemperature rise. Specifically, in the battery charger according to theconventional technique, the battery is charged with current of aconstant value. In the battery charger of this embodiment, by contrast,the state of the battery is determined based on the absolute temperatureand temperature rise, the battery is charged with as high current aspossible which can be applied while suppressing the battery temperaturefrom rising, i.e., the battery is charged with varying current inaccordance with the state of the battery.

In this embodiment, if the battery temperature is high, relatively lowcharging current is applied. If the temperature is low, relatively highcharging current is applied. If temperature rise is large, relativelylow charging current is applied. If the temperature rise is small,relatively high charging current is applied.

The map is provided for variable-control of current and for specifyingan optimum value of current which can be applied when temperature riseis suppressed. In the map, the horizontal axis indicates the absolutetemperature T of a battery and the vertical axis indicates a temperaturedifference dT/dt. That is, if the temperature of the battery is high andtemperature rise is large (lower right side on the map), then arelatively low charging current is applied. If the temperature of thebattery is high and the temperature rise is small (upper end side on themap), a medium level charging current is applied. If the batterytemperature is low and the temperature rise is large (lower left side onthe map), a medium level charging current is applied. If the batterytemperature is low and the temperature rise is low (upper left on themap), a relatively high charging current is applied. Namely, suchoptimum current values are set on the map in order to meet both expectedcharging time (approximately 20 minutes) and expected final temperature.

If high current is discharged at low temperature (0° C. or less),battery performance deteriorates. It is, therefore, desirable to set alow current value on the left column on the map in order to avoidperformance deterioration.

Based on the map, a region is retrieved to meet the absolute temperatureT and a temperature difference dT/dt from the battery and chargingcurrent, it is controlled on the basis of the current specified in theregion. If, for example, the battery temperature is between T3 and T4and the battery temperature difference (charging temperature) is betweenX1 and X2, then current in the region I24 is outputted.

Thereafter, the battery charger in this embodiment detects that chargingis completed based on the movement of current within the regions on themap. That is, according to the conventional technique, completion ofcharging is detected by keeping charging current constant and observingtemperature or voltage. More specifically, the conventional devicedetects a temperature rise and a voltage variation as well as that thebattery is fully charged and voltage decreases. It also determines thatthe battery is fully charged. In the battery charger in this embodiment,since charging current is changed as stated above, it is impossible todetect that the battery charging is completed from only monitoringtemperature and a temperature variation, or voltage and a voltagevariation. In this embodiment, therefore, the battery charger detectscharging is completed based on the movement of current in ranges on themap.

While the battery is being charged, a current value apparently moveswithin the regions at random based on the temperature and temperaturerise. In other words, before the battery is fully charged, the batterytemperature rises or a temperature rise is large. If a relatively smallcharging current region is selected, that is, after a region on thelower right side on the map is selected, then current decreases andtemperature rise is smaller. The current region thus corresponds to anupper side region on the map.

Nonetheless, as the battery is close to a fully charged state, atemperature rise becomes larger due to the characteristics of the nickelmetal hydride battery. That is, as temperature rise is larger, a lowerregion is selected on the map. In that case, even if the battery ischarged with relatively low current, temperature rise remains large.Using this principle, the battery charger in this embodiment conductsmeasurements repeatedly (for example three times) at predeterminedintervals (e.g., at several thousand seconds' intervals). In a casewhere current belong to hatched regions I31, I32, I33, I34 and I35 inwhich temperature rise is large, and to region I25 in which temperatureis high and temperature rise is at medium level, it is determined thatthe battery charging is completed and the charging is then stopped.

Charging operation of the charging circuit 30 will be described in moredetail with reference to a graph shown in FIG. 6.

In FIG. 6, the horizontal axis indicates charging time, whereas thevertical axis indicates charging current and battery temperature. FIG. 6also shows, as comparison, temperature rise in a case where a nickelmetal hydride battery has been charged for one hour and temperature risein a case where the battery is rapidly charged as well as the case ofthe battery charger in this embodiment.

Conventionally, to avoid generating heat while a nickel metal hydride isbeing charged, IC charging is conducted as shown in dash line e of FIG.6. That is, a 2AH nickel metal hydride battery is charged with 2Acharging current for about one hour (65 minutes in FIG. 6). In case of1C charging, charging starts at a temperature of 20° C. as shown indotted line f of FIG. 6 and ends at a temperature of 40° C. Due to thecharacteristics of the nickel metal hydride battery, a temperature riseis seen just before the completion of charging (55 minutes after thestart of charging) shown in f′ of FIG. 6 and further temperature rise isseen from a time f″ at which charging is completed (overshoot f0). It isconsidered that the overshoot of the nickel metal hydride batterydepends on the gradient of temperature rise at the time charging iscompleted. If a gradient at f′-f″ is small, overshoot-inducedtemperature rise is small. If the gradient is large, that is,temperature greatly increases at the end of charging, thenovershoot-induced temperature rise is large.

Meanwhile, chain line c indicates current if a battery is rapidlycharged (4.5C charging) with constant high current (9A) by aconventional battery charger so as to complete charging in approximately20 minutes. Chain line d indicates the temperature change of the nickelmetal hydride in rapid charging. As indicated by the chain line d, evenif charging starts at a temperature of 20° C., the temperature reaches70° C. at which the nickel metal hydride battery life is shortened whencharging is completed. Moreover, temperature rapidly rises just beforethe completion of charging (11 minutes after start) indicated by d′ inFIG. 6 until d″ at which charging is completed. Due to this, temperaturegreatly rises at time d″ at which charging is completed (overshoot d0).The overshoot d0 causes temperature to rise further as high as 80° C.even after charging is completed, as a result the life of the nickelmetal hydride battery is shortened. According to the graph of FIG. 6,since charging starts at 20° C. and is completed at 80° C., atemperature rise is 60° C. during that period. If, therefore, chargingof the nickel metal hydride battery starts at 30° C., and temperaturerises by 60° C. to 90° C. or higher, the battery performance greatlydeteriorates at this point.

Full line <a> indicates a change in charging current in the batterycharger in the first embodiment according to the present invention,whereas full line <b> indicates a change in the temperature of nickelmetal hydride battery if the battery is charged by the battery chargerin this embodiment. The battery charger 10 in this embodiment appliesrelatively low charging current if battery temperature is high andtemperature rise is large. The device 10 applies medium level chargingcurrent if battery temperature is high and temperature rise is small.The device 10 applies medium level charging current if batterytemperature is low and temperature rise is large. The device 10 appliesrelatively high charging current if battery temperature is low andtemperature rise is small. In this way, the battery charger 10 in thisembodiment adjusts current based on the temperature of the nickel metalhydride battery and its temperature rise. The device 10 starts chargingthe battery at a temperature of 20° C. as shown in the full line <b>while controlling the temperature to fall within 50° C. or less so asnot to affect the battery life. In other words, the device 10 adjustscharging current to maximum current while avoiding exceeding theexpected temperature and shortening charging time.

As stated above, the battery charger 10 constantly changes chargingcurrent in accordance with the battery temperature and temperature rise.That is, high current is applied at an initial charging stage, i.e.,while battery temperature is low and temperature rise is small.Relatively low charging current is applied at a final charging stage,i.e., if battery temperature is high and temperature rise is large sothat temperature rise is kept small just before the completion ofcharging. Specifically, temperature rise is small (or temperature risegradient is small) from a time b′ (11 minutes after the start ofcharging) just before the completion of charging until a time b″ atwhich charging is completed. As a result, temperature rise (overshootb0) after the completion of charging is small and heat generation duringand after charging is suppressed such that temperature rise during andafter charging is almost as same (approximately 50° C.) as that in 1Ccharging indicated by the dotted line f.

Even in 1C charging as indicated by the dotted line f, if batterytemperature at the start of charging is high, for example, if chargingis started at a battery temperature of 30° C., then temperature rises by30° C. and reaches 60° C. at the completion of charging. In thisembodiment, by contrast, current is controlled in accordance withtemperature and battery temperature can be limited to up to 50° C. whencharging is completed. In the case of the power tool battery, inparticular, where the motor is continuously driven with high current andcapacity is exhausted, it is probable that charging starts when batterytemperature is high enough. In the battery charger in this embodiment,even if a nickel metal hydride battery is at such a high temperature itcan charge to the extent that the temperature does not exceed anexpected charging temperature. It is, thus, possible to repeatedly usethe nickel metal hydride battery for a long period of time.

Specific processing of the battery charger in this embodiment will bedescribed with reference to FIGS. 5 and 6 as well as the flow chartshown in FIG. 7.

First, after charging starts, a control section 36 of the batterycharger (see FIG. 4) adjusts charging current and determines whetherbattery charging is completed in a predetermined cycle (here, at 100seconds' intervals for the convenience of description, but actually farshorter intervals of 10 seconds or less). Specifically, first, theabsolute temperature T of the nickel metal hydride battery is inputted(in step S12). Next, the inputted absolute temperature T isdifferentiated and a battery temperature difference dT/dt is calculated(in step S14). Based on the absolute temperature T and the temperaturedifference dT/dt, an optimum charging current is selected from the mapwhich has been already described with reference to FIG. 5 (in step S16).In this case, as indicated in cycle <1>, where the absolute temperatureT is between T1 and T2 and the temperature difference dT/dt is X1 orless, region I12 is selected and relatively high current of 4.5Ccharging current (9A) is applied as indicated by the full line a shownin FIG. 7.

Thereafter, the control section 36 determines whether current is infinal charging regions, i.e., hatched regions I31, I32, I33, I34 and I35in which temperature rise is large and in a region I25 in whichtemperature rise is medium level (in step S18). In this case, since thecurrent is not in the final charging region (“NO” in the Step S18),process returns to the step S12 and control of the charging currentcontinues. In cycle <2> after more than 100 seconds pass, because arelatively high current has been applied in cycle <1> as stated above,the temperature difference dT/dt is larger (X1 to X2) and region I22 isselected, so that medium level current (3.5C) is selected. Since themedium level current is selected in the cycle <2>, the temperaturedifference dT/dt turns into X1 or less. In cycle <3>, the region I12 isselected and relatively high charging current is thereby selected onceagain.

As stated above, if charging is continued while current value is changedin accordance with the absolute temperature T and the temperaturedifference dT/dt, the difference dT/dt gradually increases. In cycle<6>, as shown in FIG. 5, the temperature difference dT/dt exceeds X2 andenters region I33. In this case, the determination result is Yes in step18. That is, current is determined in the final charging regions(regions I31, I32, I33, I34, I35 and I25) as stated above, and it isthen determined whether current is highly probable in the final chargingregions (in step S22). It is determined that it is highly probable inthe final charging regions if it enters the final charging regions inthree consecutive cycles. While the current is decreased in the cycle<6>, the absolute temperature T is between T2 and T3 and the temperaturedifference dT/dt is decreased to X1 to X2 in the next cycle <7>, wherebycurrent is in the region I23. Thus, the determination result as towhether it is highly probable that current is in the final chargingregion is Low in step S22 and the process goes on to step 20 in whichcharging continues with a variable charging current.

Meanwhile, if current enters region I25 which belongs to final chargingregions in cycle <13>, current enters the region I35 which also belongsto the final charging regions in the next cycles <14> and <15>. Ifcurrent enters final charging regions in three consecutive cycles, thedetermination result as to whether it is high probable that currententers a final charging region is High and charging is completed (instep S24), thereby ending all series of processing.

In the above-stated example, description has been given to a case wherecycle time is 100 seconds for convenience of description. Due to this,it is determined that it is highly probable if current enters finalcharging regions in three consecutive cycles. If cycle time is madeshorter than a 100 seconds' cycle, it is possible to determine that itis high probable that current enters final charging regions usingvarious methods. For instance, in a 10 seconds' cycle, it is highlyprobable that current enters the final charging regions in eight out of10 cycles. Alternatively, it can be determined that probability is highif current enters the final charging regions in eight out of 10 cyclesand if current enters the final charging regions in five consecutivecycles.

As described above, the conventional battery charger for charging anickel-cadmium battery detects that charging is completed by keeping acurrent value constant and monitoring one or more of temperature,temperature change, voltage and voltage change. In the case of a nickelmetal hydride battery, however, patterns of temperature and voltagechange vary in accordance with the battery remaining capacity, batterytemperature at the start of battery charging and outside temperature.Due to this, it has been difficult to fully charge the nickel metalhydride battery without fear of overcharge. The battery charger in thisembodiment, by contrast, continuously monitors the absolute temperatureT and temperature difference dT/dt while narrowing down chargingcurrent. It is, therefore, possible to fully charge the battery withoutfear of overcharge.

The embodiment has been described so far, taking processing for charginga nickel metal hydride battery for an example. The battery charger andcharging method according to the present invention can be also appliedto processing for charging a nickel-cadmium battery. As specified, ifthe map shown in FIG. 5 is adjusted to the characteristics of thenickel-cadmium battery, the present invention is applicable to theprocess for charging a nickel-cadmium battery. In addition, if the mapis adjusted to the common characteristics (greatest common divisor) of anickel-cadmium battery and a nickel metal hydride battery, both thenickel-cadmium battery and the nickel metal hydride battery can becharged. Furthermore, the battery pack 50 may be provided with adiscriminating terminal (such as a boss) for discriminating a nickelmetal hydride battery from a nickel-cadmium battery and a map may beprepared for each of the nickel metal hydride battery and thenickel-cadmium battery at the battery charger side. If the battery pack50 of the nickel metal hydride battery is installed, charging can becontrolled according to the map for the nickel metal hydride battery. Ifthe battery pack 50 of the nickel-cadmium is installed, charging can becontrolled according to the map for the nickel-cadmium battery.

As stated so far, according to the present invention, the map isretrieved and the battery is charged with allowable current with whichthe battery can be charged while suppressing battery temperature fromrising. This makes it possible to charge a nickel metal hydride battery,the temperature of which tends to rise during charging, for a shortperiod of time without causing a deterioration in the battery due totemperature rise. In addition, just before the completion of charging,the temperature rise of the nickel metal hydride battery is larger andthe battery is charged with a relatively low current, so that it ispossible to suppress “overshoot” after the completion of charging.

According to the present invention, the map is retrieved and the batteryis charged with allowable current with which the battery can be chargedwhile suppressing battery temperature from rising. This makes itpossible to charge a nickel metal hydride battery, the temperature ofwhich tends to rise during charging, for a short period of time withoutcausing deterioration in the battery due to temperature rise. Inaddition, just before the completion of charging, the temperature riseof the nickel metal hydride battery is larger and the battery is chargedwith relatively low current, so that it is possible to suppress“overshoot” after the completion of charging.

The completion of charging is determined, in particular, based onwhether or not temperature rise is large and whether or not temperaturerise is large even if the charging current is lowered. Due to this, itis possible to fully charge the battery without fear of overcharge andwithout being influenced by the remaining battery capacity, temperatureand the like.

What is claimed is:
 1. A battery charger comprising: a memory forstoring a map for mapping an allowable value of current, with which abattery can be charged while suppressing battery temperature fromrising, based only on a battery temperature and a battery temperaturerise; a temperature detection device for detecting a current temperatureof the battery; a temperature rise output device for obtaining thetemperature rise from the temperature detected by said temperaturedetection device; an allowable current retrieval device for retrievingthe map of said memory device from the temperature detected by saidtemperature detection device and the temperature rise outputted fromsaid temperature rise output device, and for obtaining said allowablevalue; and a charging device for charging the battery with the allowablecurrent retrieved by said allowable current retrieval device.
 2. Thebattery charger according to claim 1, wherein said allowable currentretrieval device determines, after said charging device starts chargingthe battery, if the obtained current value is within a predeterminedportion of the map representing predetermined levels of temperaturerise, and said charging device continues charging the battery until thecurrent value is within the predetermined portion.
 3. The batterycharger according to claim 2, wherein said predetermined levels aremiddle and high levels of temperature rise.
 4. A charging methodcomprising: detecting current temperature of a battery; obtaining atemperature rise from said detected temperature; retrieving an allowablecurrent map based only on battery temperature and battery temperaturerise, from said detected temperature and said obtained temperature rise,and obtaining an allowable value of current, with which the battery canbe charged while suppressing the battery temperature from rising; andcharging the battery with said retrieved allowable current.
 5. Thecharging method according to claim 4, wherein after starting to chargethe battery, determining if said allowable current is within apredetermined portion of the map representing predetermined levels oftemperature rise, and continuing charging the battery until the currentvalue is within the predetermined portion.
 6. The charging methodaccording to claim 5, wherein said predetermined levels are middle andhigh levels of temperature rise.